Police also detained a leading opposition leader Vladimir Nekliayev as he was returning from Poland, taking him off the train at the border and placing him in a detention facility.
Viasna, an NGO that had been tracking arrests and protest rallies in Belarus in recent weeks, said riot police blocked their office in Minsk and was conducting a search.
The police broke down the door, "put people face down on the floor and told them to stay there," the NGO said on its website. "There were 57 people detained, including foreign observers," it said.
A protest had been scheduled by the opposition for 2:00 PM in Minsk and several other cities, the latest in a series of events against authoritarian President Alexander Lukashenko.
Thousands have attended rallies in recent weeks to oppose a controversial new tax on "spongers" - those who work less than six months a year - as the country suffers an economic slump, with the swell in protests alarming the government.
Authorities late yesterday told organisers that the event would be illegal. Today, scores of armoured police trucks and water cannons, as well as officers armed with automatic rifles could be seen in the city.
Opposition leader Nekliayev, who was set to speak at the protest, was stopped at the border this morning on his way to Minsk, his wife told AFP.
"He is in a detention facility in Brest," Olga Nekliayeva said, referring to the city in southwestern Belarus close to the Polish border.
Dozens have been arrested ahead of today's event, as state television aired reports of alleged weapons caches discovered while police armed with automatic rifles were in the city centre for the first time in decades.
Belarusian independent newspaper Nasha Niva added that prominent opposition leader Mikola Statkevich and his wife Marina Adamovich had their Facebook accounts hacked, citing Adamovich's son Yuri.
Statkevich's Facebook account was offline today while his official website late yesterday posted that "the rally has been cancelled" in what his supporters also believed to be a fake.
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